Problem: Convert the point $(1,-\sqrt{3})$ in rectangular coordinates to polar coordinates.  Enter your answer in the form $(r,\theta),$ where $r > 0$ and $0 \le \theta < 2 \pi.$
Explanation: We have that $r = \sqrt{1^2 + (-\sqrt{3})^2} = 2.$  Also, if we draw the line connecting the origin and $(1,-\sqrt{3}),$ this line makes an angle of $\frac{5 \pi}{3}$ with the positive $x$-axis.

[asy]
unitsize(0.8 cm);

draw((-2.5,0)--(2.5,0));
draw((0,-2.5)--(0,2.5));
draw(arc((0,0),2,0,300),red,Arrow(6));
draw((0,0)--(1,-sqrt(3)));

dot((1,-sqrt(3)), red);
label("$(1,-\sqrt{3})$", (1,-sqrt(3)), NE);
dot((2,0), red);
[/asy]

Therefore, the polar coordinates are $\boxed{\left( 2, \frac{5 \pi}{3} \right)}.$